It is proposed to study by in vitro culture methods the kinetics of the morphologically unidentifiable intermediate erythroid precursors and their regulation by erythropoietin. BFU-E and CFU-E will be assayed as a measure of the early and late precursors in the bone marrow of mice under the following conditions: 1) Chronic inflammation induced by turpentine abcesses. In preliminary experiments the number of marrow CFU-E was found to be markedly reduced, and the proposed studies should decide whether and to what extent the BFU-E are also affected and whether the decrease in erythroid precursors is due to decreased erythropoietin formation or to other factors; 2) Erythropoietin markedly enhances erythroid recovery in BCNU-treated mice, and the model will be used to assess the stimulatory effect of Epo on the generation and proliferation on BFU-E, and on their progression into CFU-E. A third project deals with the difference of in vivo and in vitro effects of erythropoietin on CFU-E recruitment. In vivo, its injection increases within 16 hours the femoral marrow CFU-E in posthypoxic polycythemic mice by 800%, but addition of higher concentrations of erythropoietin to the culture of marrow cells from polycythemic mice has no effect on the number of erythroid colonies. Proposed experiments should decide whether this lack of in vitro action of Epo on recruitment of CFU-E is attributable to culture conditions or whether other factors are involved which are possibly related to the in vivo hematopoietic microenvironment. Other experiments including cell separation by velocity sedimentation, are designed to obtain further information on cell phase of CFU-E and their position in erythroid cell development.